Process for fabricating a multilayer ceramic circuit board

ABSTRACT

A process for fabricating a multilayer ceramic circuit board which includes the steps of (i) providing a sintered ceramic substrate having optionally formed via holes already filled with a conductive paste or a conductive substance, (ii) optionally applying an insulating paste layer in a prescribed pattern on one side or each side of the ceramic substrate, (iii) then, providing a ceramic green tape and forming via holes in the green tape, (iv) applying a conductive paste to form a conductive pattern on one side of the green tape, (v) laminating the green tape to one side or each side of the ceramic substrate so that the conductive pattern of the green tape faces the substrate, (vi) optionally pressing and heating the laminate from step (v), and (vii) applying a conductive paste to form a conductive pattern on the other side of the green tape and filling the via holes of the green tape with a conductive paste, before or after step (v).

This is a continuation of application Ser. No. 07/589,215, now U.S. Pat.No. 5,176,772, filed on Sep. 28, 1990.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for fabricating a multilayerceramic circuit board, wherein a green tape is laminated on a sinteredceramic substrate, followed by sintering, so as to avoid a distortion ofthe laminate due to shrinkage of the green tape during the sintering.

2. Discussion of Background

As a method for fabricating a multilayer ceramic circuit board havingexcellent strength, a method has been reported which compriseslaminating a green tape on one side of a sintered ceramic substratehaving a conductive pattern formed on its surface, then printing aconductive pattern on the surface of the green tape, and furtherlaminating a green tape thereon (U.S. Pat. Nos. 4,645,552 and4,799,984). However, this method requires sintering each time when agreen tape is laminated. Accordingly, the number of process stepsincreases. The conductive pattern formed on the sintered ceramicsubstrate is raised from the surface of the ceramic substrate, and thegreen tape laminated thereon will have irregularities on its surface,whereby it will be difficult to form a fine conductive pattern on such agreen tape. Further, if a defect is found in a green tape afterlamination, such a tape can not be replaced, and the entire assembly ofthe sintered ceramic substrate and the green tape laminated thereon willbe wasted. Thus, the productivity used to be poor.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the above drawbacksof the prior art and to provide a novel process for fabricating amultilayer ceramic circuit board.

The present invention has been made to accomplish the above object andprovides a process for fabricating a multilayer ceramic circuit boardwhich comprises the steps of:

(i) providing a sintered ceramic substrate having optionally formed viaholes already filled with a conductive paste or a conductive substance;

(ii) optionally applying an insulating paste layer in a prescribedpattern on one side or each side of the ceramic substrate;

(iii) then, providing a ceramic green tape and forming via holes in thegreen tape;

(iv) applying a conductive paste to form a conductive pattern on oneside of the green tape;

(v) laminating the green tape to one side or each side of the ceramicsubstrate so that the conductive pattern of the green tape faces thesubstrate;

(vi) optionally pressing and heating the laminate from step (v); and

(vii) applying a conductive paste to form a conductive pattern on theother side of the green tape and filling the via holes of the green tapewith a conductive paste, before or after step (v).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross sectional view illustrating the basic structure of amultilayer ceramic circuit board prepared by the process of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in detail with reference tothe drawing.

FIG. 1 is a cross sectional view illustrating the basic structure of amultilayer ceramic circuit board prepared by the process of the presentinvention.

In FIG. 1, reference numeral 1 indicates a sintered ceramic substrate.As the material for the substrate, various ceramics, specificallyalumina (Al₂ O₃), beryllia, aluminum nitride (AlN), magnesia (MgO),glass ceramics, etc., may usually be employed. Among them, from theviewpoint of strength, an alumina substrate containing from 88 to 99.5wt % of alumina is preferred, and from the viewpoint of heatconductivity, AlN or beryllia is preferred.

Reference numerals 2 and 3 indicate green tape layers formed bylamination on the above ceramic substrate 1.

As the material for the green tape layers 2 and 3, various ceramics,specifically alumina, beryllia, AlN, MgO, glass ceramics, etc. mayusually be employed, and the sintering temperature for the material ispreferably lower than that for the ceramic substrate 1. If the sinteringtemperature for the green tape layers 2 and 3 is higher than that forthe ceramic substrate 1, the ceramic substrate 1 is likely to bedeformed at the time of sintering, such being undesirable. Accordingly,it is preferred to adopt a combination of a ceramic substrate 1 made ofalumina and green tape layers 2 and 3 made of ceramics of borosilicateglass type, alumina-glass type, citric acid titanium tin barium type oralumina-non glass additive type, which has a sintering temperature lowerthan that for alumina.

Further, the strength of the material of the ceramic substrate 1 ispreferably stronger than the material for the green tape layers 2 and 3.When the strength of the material for the ceramic substrate 1 issubstantially the same as the substrate for the green tape layers 2 and3, the thickness of the ceramic substrate 1 should be thicker than thethickness of the green tape layers 2 and 3.

Reference numerals 4 and 5 indicate conductive paste layers. Theconductive paste to be used here is suitably selected as the caserequires, for example, from copper (Cu), silver (Ag), Ag-palladium (Pd),gold (Au), platinum (Pt) and tungsten (W).

Now, the process for fabricating a multilayer ceramic circuit boardaccording to the present invention will be described.

(i) Firstly, a sintered ceramic substrate 1 is provided which isoptionally provided with via holes 6 already filled with a conductivepaste or a conductive substance.

(ii) An insulating paste layer 7 is formed in a prescribed pattern onone side or each side of the ceramic substrate 1 by e.g. a printingmethod.

(iii) Then, a ceramic green tape 2 is provided, and via holes are formedin the green tape.

(iv) A conductive paste is applied to form a conductive pattern on oneside of the green tape by e.g. a printing method.

(v) The green tape 2 is laminated to one side or each side of theceramic substrate 1 so that the conductive pattern of the green tape 2faces the substrate 1.

(vi) The laminate thus formed is pressed and heated, as the caserequires.

(vii) A conductive paste is applied to form a conductive pattern on theother side of the green tape 2, and filling the via holes of the greentape with a conductive paste, before or after step (v). Namely, thisstep may be conducted before the lamination i.e. at the same time asstep (iv), or may be conducted after the lamination i.e. after step (v).

(viii) When a further circuit layer is desired, a ceramic green tapehaving via holes formed, is laminated to the green tape on the ceramicsubstrate, and the laminate thereby obtained may optionally be pressedand heated.

(ix) Then, a conductive paste is applied to form a conductive pattern onthe green tape laminated in step (viii) and to fill the via holes of thesame green tape.

(x) Steps (viii) and (ix) may optionally be repeated until the desirednumber of circuit layers has been obtained.

(xi) Finally, the assembly of the ceramic substrate and the green tapesmay be pressed under a pressure of from about 1 to 200 kg/cm² and heatedat a temperature of from room temperature to 100° C., followed bysintering, as the case requires.

Among above steps (i) to (xi), steps (ii) may be omitted, although it isusually preferred to include such a step to improve the adhesion of thegreen tape to the ceramic substrate 1.

Now, another embodiment of the process for producing a ceramic circuitboard according to the present invention will be described.

(i) A sintered ceramic substrate 1 is provided which may be providedwith via holes already filled with a conductive paste or a conductivesubstance.

(ii) An insulating paste layer 7 is formed in a prescribed pattern onone side or each side of the ceramic substrate 1 by e.g. a printingmethod.

(iii) A conductive paste is applied in a prescribed pattern 5 on theinsulating paste layer 7 on the ceramic substrate 1 by e.g. a printingmethod.

(iv) Then, a ceramic green tape 2 is provided, and via holes are formedin the green tape.

(v) The green tape 2 is laminated to one side or each side of theceramic substrate 1.

(vi) The laminate thereby obtained may be pressed and heated, as thecase requires.

(vii) A conductive paste is then applied to form a conductive pattern onthe green tape, and the via holes of the green tape are filled with aconductive paste, before or after step (v). Namely, this step can beconducted before the lamination i.e. at the same time as step (iv), orafter the lamination i.e. after step (v).

(viii) If a further circuit layer is required, a ceramic green tape 3having via holes formed, is laminated to the green tape 2 on the ceramicsubstrate 1, and the laminate thereby obtained may be pressed and heatedas the case requires.

(ix) A conductive paste is applied to form a conductive pattern 4 on thegreen tape 3 laminated in step (viii) and to fill the via holes of thesame green tape 3.

(x) Steps (viii) and (ix) may be repeated until the desired number ofcircuit layers has been obtained.

(xi) Finally, the assembly of the ceramic substrate and the green tapesmay be pressed under a pressure of from about 1 to 200 kg/cm² and heatedat a temperature of from room temperature to 100° C., followed bysintering, as the case requires.

Now, a preferred material for the green tape will be described. In thefollowing description, "%" means "% by weight".

The green tape comprises an inorganic component comprising from 40 to90% of glass powder, from 9.99 to 60% of refractory filler and from 0.01to 20% of an oxidizing agent. If the content of the glass powder in theinorganic component is less than 40%, the adhesion to the ceramicsubstrate 1 tends to be poor, and an adequately dense sintered layertends to be hardly obtainable, whereby the electric properties tend tobe poor. On the other hand, if the content exceeds 90%, the reactivitywith the conductor such as copper tends to increase, and the wettabilityof the conductor such as copper to solder tends to be impaired, suchbeing undesirable.

The refractory filler powder is incorporated to improve the peelstrength of the conductive pattern. If the content is less than 9.99%,this peel strength tends to be poor, and if the content exceeds 60%, thesintering properties tend to be poor.

As the refractory filler, alumina alone, or a mixture of alumina (Al₂O₃), zircon (ZrSiO₄), cordierite (Mg₂ Al₄ Si₅ O₁₈), forsterite (Mg₂SiO₄) and silica (SiO₂), may be used.

In the present invention, the oxidizing agent is incorporated to oxidizeand remove the organic binder in the composition for forming a circuitboard by firing in a non-oxidizing atmosphere at an oxygen concentrationof not higher than about 10 ppm (binder-removing effects).

The oxidizing agent is added preferably in an amount of from 0.01 to 20%by weight relative to the inorganic component i.e. the total amount ofthe glass powder and the refractory filler. If the amount of theoxidizing agent is less than 0.01%, no adequate effects by theincorporation will be obtained. On the other hand, if it exceeds 20%,the breakdown voltage of the substrate tend to deteriorate, and theoxidizing effects during sintering against the laminated circuitportions of a conductor such as copper tend to be so strong that suchcircuit portions will be oxidized, such being undesirable.

As such an oxidizing agent, CeO₂, TiO₂, BaO₂, SnO₂, CaO₂ or V₂ O₅ may beused. CeO₂ is particularly preferred.

The glass powder preferably has the following composition:

    ______________________________________                                        composition:                                                                  ______________________________________                                        SiO.sub.2         38-48%                                                      Al.sub.2 O.sub.3  1-8%                                                        MgO                0-10%                                                      CaO               1-8%                                                        SrO                0-15%                                                      BaO               18-28%                                                      PbO                0-20%                                                      ZnO               10-20%                                                      B.sub.2 O.sub.3   0.5-15%                                                     TiO.sub.2 + ZrO.sub.2                                                                           0-7%                                                        Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                             0-5%                                                        ______________________________________                                    

The reason is as follows.

If the amount of SiO₂ is less than 38%, the dielectric constant of thesintered substrate tends to be too large, and if it exceeds 48%, thesintering temperature tends to be too high.

If the amount of Al₂ O₃ is less than 1%, the moisture resistance of thesintered substrate tends to be poor, and if it exceeds 8%,devitrification is likely to result during the preparation of glassfrit.

CaO and BaO are components for controlling the solubility and thethermal expansion coefficient for the preparation of the glass powder.The solubility of the glass powder tends to be poor if the content ofCaO or BaO is less than 1% or 18%, respectively. On the other hand, ifCaO or BaO exceeds 8% or 28%, respectively, the thermal expansioncoefficient and the dielectric constant of the substrate tend to be toolarge, such being undesirable.

MgO and SrO have substantially the same effect as CaO and BaO. If MgO orSrO exceeds 10% or 15%, respectively, devitrification is likely toresult when glass is melted.

By the addition of PbO, the glass softening temperature lowers, and thesintering temperature of the substrate lowers. However, if the amountexceeds 20%, the dielectric constant of the substrate tends to be toolarge, and the removal of the organic binder tends to be inadequate andcarbon is likely to remain, such being undesirable.

B₂ O₃ is a flux component. If the amount is less than 0.5%, no adequateeffect will be obtained as a flux. If the amount of B₂ O₃ exceeds 15%,the removal of the organic binder tends to be inadequate, and carbon islikely to remain, although the dielectric constant decreases and theelectrical properties will be improved.

By the addition of ZnO, the glass softening temperature lowers, and thesintering temperature of the substrate lowers, such being desirable.However, if the amount exceeds 20%, the dielectric constant of thesubstrate increases, and if the amount is less than 10%, the glasssoftening temperature tends to be high, such being undesirable.

TiO₂ +ZrO₂ are not essential components. However, by their addition, thecrystallinity can advantageously be adjusted. If the amount exceeds 7%,the softening temperature of glass tends to be too high, or thedielectric constant of the substrate tends to be large.

Li₂ O+Na₂ O+K₂ O are not essential components, but they are preferablyemployed for the improvement of the solubility of glass. However, fromthe viewpoint of electrical migration, the amount is preferably lessthan 5%.

The above-mentioned refractory filler preferably has the followingcomposition expressed by % by weight:

    ______________________________________                                               Alumina 10-60%                                                                Zircon  0-40%                                                                 Cordierite                                                                            0-30%                                                                 Forsterite                                                                            0-30%                                                                 Silica  0-30%                                                          ______________________________________                                    

The reason is as follows. If alumina is less than 10%, the crystallinityof glass tends to be low, and if it exceeds 60%, the sinteringproperties tend to deteriorate, and the dielectric constant tends toincrease.

Zircon is added to reduce and minimize the thermal expansion. However,if it exceeds 40%, the sintering properties tend to be poor.

Cordierite is incorporated to minimize the thermal expansion and tominimize the dielectric constant. However, if it exceeds 30%, thesintering properties tend to be poor.

Forsterite is incorporated to minimize the dielectric constant. However,if it is added in an amount exceeding 30%, the sintering properties tendto be poor.

Silica is added to minimize the dielectric constant. However, if itexceeds 30%, the sintering properties tend to be poor.

The green tape for the present invention may be prepared, for example,as follows.

An organic binder, a plasticizer and a solvent are added to the abovedescribed inorganic component, and the mixture is kneaded to obtain aslurry. Here, as the organic binder, a commonly employed resin such as abutyral resin or an acrylic resin may be employed. As the plasticizer, acommonly employed plasticizer such as dibutyl phthalate, dioctylphthalate or butyl benzil phthalate, may be used; and as the solvent, acommonly employed solvent such as toluene, trichloroethylene or analcohol may be employed.

Then, this slurry is formed into a sheet, followed by drying to obtain anon-sintered so-called green tape.

The thickness of this green tape is usually from 70 to 300 μm.

Now, the insulating paste for the present invention will be described.The inorganic component for the insulating paste preferably has the samecomposition as the above described green tape having good adhesion tothe ceramic substrate 1. However, it is not limited to such acomposition, and other compositions may be used.

The insulating paste may be prepared, for example, as follows. Anorganic binder and a solvent are added to the same inorganic compositionas used for the above described green tape, and the mixture is kneadedto obtain an insulating paste. Here, as the organic binder, ethylcellulose, a vinyl acetate resin or an acrylic resin may be used, and asthe solvent, α-terpineol or butyl carbitol may be employed.

Such an insulating paste is applied onto the above described ceramicsubstrate in a thickness within a range of from 5 to 50 μm by e.g.screen printing.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted to such specific Examples.

EXAMPLE 1

A glass ceramic composition comprising 58% of glass frit, 40% of aluminaas filler and 2% of CeO₂ as oxidizing agent, was prepared. The glassfrit had a composition comprising 43% of SiO₂, 5% of Al₂ O₃, 5% of CaO,27% of BaO, 15% of ZnO and 5% of B₂ O₃.

To this glass ceramic composition, a methyl methacrylate resin asorganic binder, dibutyl phthalate as plasticizer and toluene as solventwere added, and the mixture was kneaded to obtain a slurry having aviscosity of from 10,000 to 30,000 cps. Then, this slurry was formedinto a sheet having a thickness of about 0.2 mm, followed by drying at70° C. for about 2 hours. To this green tape, a copper paste wasscreen-printed to obtain patterns with three different line widths andspaces between lines, of 100 μm, 125 μm and 150 μm, respectively.

Then, a substrate of 96% alumina having a thickness of 0.635 mm wasprepared. Via holes formed in this alumina substrate were filled withthe above copper paste. Then, an insulating paste was prepared bykneading the same inorganic component as in the above green tape, ethylcellulose as organic binder and α-terpineol as solvent, and theinsulating paste was printed on each side of the alumina substrate in athickness of 20 μm.

Then, the above green tape was laminated to each side of the ceramicsubstrate 1 so that the copper paste layer on the green tape faces thesubstrate, to obtain a laminate.

Further, the same green tape as described above is laminated to eachside of this laminate. Then, a copper paste was printed on this greentape in the same manner as described above.

Then, the assembly thereby obtained was heat-pressed at 70° C. under 60kg/cm² and sintered in a nitrogen atmosphere having an oxygenconcentration of 5 ppm and a flow rate of 20 l/min at the maximumtemperature of 900° C. for a peak time of 10 minutes. In this manner,1,000 multilayer ceramic circuit boards were prepared.

With respect to all of the 1,000 multilayer ceramic circuit boards,warpage, short circuit, opencircuit, etc. were inspected, whereby nodefective sheet was found.

Here, evaluation was made on the basis such that impedance between thepatterned lines being not higher than 10¹⁴ was regarded as beingdefective.

EXAMPLE 2

Multilayer ceramic circuit boards were prepared in the same manner as inExample 1 except that the material for green tapes was as identified inTable 1. The number of boards prepared was 1,000 boards for each sampleNo. The results were the same as in Example 1, and all the products werefree from defects.

EXAMPLE 3

Multilayer ceramic circuit boards were prepared in the same manner andin the same number of products as in Example 1 and 2 except that noinsulating paste was printed to the alumina substrate.

The results were substantially the same as in Examples 1 and 2, and allthe products were free from defects.

EXAMPLE 4

Multilayer ceramic circuit boards were prepared in the same manner as inExamples 1 and 2 except that a conductive paste was printed on theinsulating paste on the alumina substrate, and no conductive paste wasapplied to the green tape for the first layer.

The results were the same as in Examples 1 and 2.

COMPARATIVE EXAMPLE 1

In Table 2, Comparative samples are shown. In this Comparative Example,1,000 multilayer ceramic circuit boards were prepared for each sampleusing the material for the green tape as identified in Table 2 in thesame manner as in Example 1. As shown in the column for the propertiesin Table 2, all the products were defective.

COMPARATIVE EXAMPLE 2

Multilayer ceramic circuit boards were prepared in the same manner andin the same number of products as in Examples 1 to 3 except that thecopper paste was printed on the ceramic substrate in three differentpatterns with line widths and spaces between the lines being 100 μm, 125μm and 150 μm, respectively, and no copper paste was printed on thegreen tape laminated first on the ceramic substrate.

The results were the same for the products of the respective sample Nos.Namely, with the pattern of 100 μm, 50% of the products were defective;with the pattern of 125 μm, 10% of the products were defective; and withthe pattern of 150 μm, all the products were free from defects. The samestandards as in Example 1 were employed for evaluation.

                                      TABLE 1                                     __________________________________________________________________________    (Examples)                                                                    __________________________________________________________________________    Sample No.  1    2    3    4    5    6    7    8    9    10                   __________________________________________________________________________    Composition (%)                                                               Alumina     8    18   28   38   48   58   20   25   38   38                   Zircon      0    0    0    0    0    0    0    0    0    0                    Forsterite  0    0    0    0    0    0    0    0    0    0                    Cordierite  0    0    0    0    0    0    0    0    0    0                    Silica      0    0    0    0    0    0    0    0    0    0                    Glass       90   80   70   60   50   40   70   50   40   40                   Glass                                                                         composition (%)                                                               SiO.sub.2   43   43   43   43   43   43   43   43   43   43                   Al.sub.2 O.sub.3                                                                          5    5    5    5    5    5    5    5    5    5                    MgO         0    0    0    0    0    0    0    0    0    0                    CaO         5    5    5    5    5    5    5    5    5    5                    SrO         0    0    0    0    0    0    0    0    0    0                    BaO         27   27   27   27   27   27   27   27   27   27                   PbO         0    0    0    0    0    0    0    0    0    0                    ZnO         15   15   15   15   15   15   15   15   15   15                   B.sub.2 O.sub.3                                                                           5    5    5    5    5    5    5    5    5    5                    TiO.sub.2 + ZrO.sub.2                                                                     0    0    0    0    0    0    0    0    0    0                    Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                       0    0    0    0    0    0    0    0    0    0                    Oxidizing                                                                     agent (%)                                                                     CeO.sub.2   2    2    2    2    2    2    2    2    2    2                    BaO.sub.2   0    0    0    0    0    0    0    0    0    0                    CaO.sub.2   0    0    0    0    0    0    0    0    0    0                    Properties                                                                    WV*.sup.1 (KV/mm)                                                                         15   17   17   20   25   22   23   24   20   20                   IR*.sup.2 (Ω)                                                                       4 × 10.sup.12                                                                4 × 10.sup.12                                                                4 × 10.sup.12                                                                5 × 10.sup.12                                                                2 × 10.sup.13                                                                9 × 10.sup.13                                                                1 × 10.sup.13                                                                2 × 10.sup.13                                                                3 × 10.sup.12                                                                5 ×                                                                     10.sup.12            DC*.sup.3   6.7  6.8  7.0  7.4  7.5  7.5  6.7  7.2  7.1  6.8                  DCT*.sup.4 (%)                                                                            0.5  0.5  0.3  0.1  0.1  0.2  0.2  0.2  0.2  0.1                  Adh*.sup.5  Good Good Good Good Good Good Good Good Good Good                 Wet*.sup.6 (%)                                                                            90   95   98   100  100  100  97   100  100  100                  Black*.sup.7                                                                              Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil                  RC*.sup.8 (ppm)                                                                           360  240  240  180  170  170  240  170  120  120                  __________________________________________________________________________    Sample No.  11   12   13   14   15   16   17   18   19   20                   __________________________________________________________________________    Composition (%)                                                               Alumina     49   49   49   49   49   49.97                                                                              49.9 49.5 49   45                   Zircon      0    0    0    0    0    0    0    0    0    0                    Forsterite  0    0    0    0    0    0    0    0    0    0                    Cordierite  0    0    0    0    0    0    0    0    0    0                    Silica      0    0    0    0    0    0    0    0    0    0                    Glass       49   49   49   49   49   50   50.0 50   50   50                   Glass                                                                         composition (%)                                                               SiO.sub.2   43   43   43   43   43   43   43   43   43   43                   Al.sub.2 O.sub.3                                                                          5    5    5    5    5    5    5    5    5    5                    MgO         0    0    0    0    0    0    0    0    0    0                    CaO         5    4    4    4    4    5    5    5    5    5                    SrO         0    0    0    0    0    0    0    0    0    0                    BaO         27   25   25   24   23   27   27   27   27   27                   PbO         0    0    0    0    0    0    0    0    0    0                    ZnO         15   20   18   14   10   15   15   15   15   15                   B.sub.2 O.sub.3                                                                           5    3    5    10   15   5    5    5    5    5                    TiO.sub.2 + ZrO.sub.2                                                                     0    0    0    0    0    0    0    0    0    0                    Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                       0    0    0    0    0    0    0    0    0    0                    Oxidizing                                                                      agent (%)                                                                    CeO.sub.2   2    2    2    2    2     0.03                                                                              0.1  0.5  1    5                    TiO.sub.2   0    0    0    0    0    0    0    0    0    0                    BaO.sub.2   0    0    0    0    0    0    0    0    0    0                    SnO.sub.2   0    0    0    0    0    0    0    0    0    0                    CaO.sub.2   0    0    0    0    0    0    0    0    0    0                    V.sub.2 O.sub.5                                                                           0    0    0    0    0    0    0    0    0    0                    Properties                                                                    WV*.sup.1 (KV/mm)                                                                         15   25   26   19   15   15   16   18   19   25                   IR*.sup.2 (Ω)                                                                       2 × 10.sup.13                                                                2 × 10.sup.13                                                                2 × 10.sup.13                                                                5 × 10.sup.12                                                                1 × 10.sup.12                                                                5 × 10.sup.11                                                                9 × 10.sup.11                                                                2 × 10.sup.12                                                                8 × 10.sup.12                                                                3 ×                                                                     10.sup.13            DC*.sup.3   7.5  7.7  7.5  7.1  6.9  7.4  7.4  7.4  7.5  7.6                  DCT*.sup.4 (%)                                                                            0.1  0.1  0.1  0.1  0.1  0.4  0.3  0.2  0.1  0.1                  Adh*.sup.5  Good Good Good Good Good Good Good Good Good Good                 Wet*.sup.6 (%)                                                                            100  100  100  100  100  100  100  100  100  100                  Black*.sup.7                                                                              Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil                  RC*.sup.8 (ppm)                                                                           170  150  170  240  270  280  250  230  200  160                  __________________________________________________________________________    Sample No.  21   22   23   24   25   26   27   28   29   30                   __________________________________________________________________________    Composition (%)                                                               Alumina     40   35   30   49   49   49   49   49   49   49                   Zircon      0    0    0    0    0    0    0    0    0    0                    Forsterite  0    0    0    0    0    0    0    0    0    0                    Cordierite  0    0    0    0    0    0    0    0    0    0                    Silica      0    0    0    0    0    0    0    0    0    0                    Glass       50   50   50   49   49   49   49   49   49   49                   Glass                                                                         composition (%)                                                               SiO.sub.2   43   43   43   43   43   43   43   43   43   43                   Al.sub.2 O.sub.3                                                                          5    5    5    5    5    5    5    5    5    5                    MgO         0    0    0    0    0    0    0    0    0    0                    CaO         5    5    5    5    5    5    5    5    5    5                    SrO         0    0    0    0    0    0    0    0    0    0                    BaO         27   27   27   27   27   27   27   27   27   27                   PbO         0    0    0    0    0    0    0    0    0    0                    ZnO         15   15   15   15   15   15   15   15   13   12                   B.sub.2 O.sub.3                                                                           5    5    5    5    5    5    5    5    5    5                    TiO.sub.2 + ZrO.sub.2                                                                     0    0    0    0    0    0    0    0    2    3                    Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                       0    0    0    0    0    0    0    0    0    0                    Oxidizing                                                                     agent (%)                                                                     CeO.sub.2   10   15   20   0    0    0    0    0    2    2                    TiO.sub.2   0    0    0    2    0    0    0    0    0    0                    BaO.sub.2   0    0    0    0    2    0    0    0    0    0                    SnO.sub.2   0    0    0    0    0    2    0    0    0    0                    CaO.sub.2   0    0    0    0    0    0    2    0    0    0                    V.sub.2 O.sub.5                                                                           0    0    0    0    0    0    0    2    0    0                    Properties                                                                    WV*.sup.1 (KV/mm)                                                                         27   30   30   18   17   17   16   16   24   25                   IR*.sup.2 (Ω)                                                                       4 × 10.sup.13                                                                1 × 10.sup.14                                                                1 × 10.sup.14                                                                1 × 10.sup.13                                                                1 × 10.sup.13                                                                8 × 10.sup.12                                                                1 × 10.sup.12                                                                1 × 10.sup.12                                                                2 × 10.sup.13                                                                3 ×                                                                     10.sup.13            DC*.sup.3   7.9  8.1  8.2  7.5  7.5  7.5  7.4  7.4  7.6  7.7                  DCT*.sup.4 (%)                                                                            0.3  0.5  0.8  0.2  0.2  0.2  0.2  0.2  0.1  0.1                  Adh*.sup.5  Good Good Good Good Good Good Good Good Good Good                 Wet*.sup.6 (%)                                                                            100  100  100  100  100  100  100  100  100  100                  Black*.sup.7                                                                              Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil                  RC*.sup.8 (ppm)                                                                           140  100  100  300  310  310  340  340  170  180                  __________________________________________________________________________    Sample No.  31   32   33   34   35   36   37   38   39   40                   __________________________________________________________________________    Composition (%)                                                               Alumina     49   49   49   49   49   49   49   49   28   38                   Zircon      0    0    0    0    0    0    0    0    0    0                    Forsterite  0    0    0    0    0    0    0    0    0    0                    Cordierite  0    0    0    0    0    0    0    0    0    0                    Silica      0    0    0    0    0    0    0    0    0    0                    Glass       49   49   49   49   49   49   49   49   70   60                   Glass                                                                         composition (%)                                                               SiO.sub.2   43   43   43   43   43   43   43   43   43   38                   Al.sub.2 O.sub.3                                                                          5    5    5    5    5    5    5    5    5    5                    MgO         0    0    0    0    0    0    0    0    0    0                    CaO         5    5    5    5    5    4    4    4    4    4                    SrO         0    0    0    0    0    0    0    0    0    0                    BaO         27   27   27   27   27   25   25   25   25   25                   PbO         0    0    0    0    0    2    5    10   10   15                   ZnO         10   8    13   12   10   18   15   10   10   10                   B.sub.2 O.sub.3                                                                           5    5    5    5    5    5    5    5    5    5                    TiO.sub.2 + ZrO.sub.2                                                                     5    7    0    0    0    0    0    0    0    0                    Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                       0    0    2    3    5    0    0    0    0    0                    Oxidizing                                                                     agent (%)                                                                     CeO.sub.2   2    2    2    2    2    2    2    2    2    2                    TiO.sub.2   0    0    0    2    0    0    0    0    0    0                    BaO.sub.2   0    0    0    0    0    0    0    0    0    0                    SnO.sub.2   0    0    0    0    0    0    0    0    0    0                    CaO.sub.2   0    0    0    0    0    0    0    0    0    0                    V.sub.2 O.sub.5                                                                           0    0    0    0    0    0    0    0    0    0                    Properties                                                                    WV*.sup.1 (KV/mm)                                                                         25   24   22   22   20   25   23   20   17   17                   IR*.sup.2 (Ω)                                                                       2 × 10.sup.13                                                                2 × 10.sup. 13                                                               9 × 10.sup.12                                                                9 × 10.sup.12                                                                5 × 10.sup.12                                                                2 × 10.sup.13                                                                7 × 10.sup.12                                                                4 × 10.sup.12                                                                3 × 10.sup.12                                                                2 ×                                                                     10.sup.12            DC*.sup.3   7.7  7.7  7.6  7.7  7.9  7.5  7.6  7.9  7.9  8.0                  DCT*.sup.4 (%)                                                                            0.1  0.1  0.2  0.2  0.3  0.1  0.1  0.1  0.2  0.2                  Adh*.sup.5  Good Good Good Good Good Good Good Good Good Good                 Wet*.sup.6 (%)                                                                            100  100  100  100  100  100  100  100  97   100                  Black*.sup.7                                                                              Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil                  RC*.sup.8 (ppm)                                                                           170  170  180  200  210  190  210  230  310  290                  __________________________________________________________________________    Sample No.  41   42   43   44   45   46   47   48   49   50                   __________________________________________________________________________    Composition (%)                                                               Alumina     48   28   38   48   58   38   38   28   49   49                   Zircon      0    0    0    0    0    10   0    10   0    0                    Forsterite  0    0    0    0    0    0    0    0    0    0                    Cordierite  0    0    0    0    0    0    10   0    0    0                    Silica      0    0    0    0    0    0    10   0    0    0                    Glass       50   70   60   50   40   50   40   60   49   49                   Glass                                                                         composition (%)                                                               SiO.sub.2   33   43   43   43   43   40   47   40   43   43                   Al.sub.2 O.sub.3                                                                          5    5    5    5    5    7    6    7    5    5                    MgO         0    0    0    0    0    0    0    0    2    5                    CaO         4    6    6    6    6    7    2    7    5    5                    SrO         0    0    0    0    0    0    0    0    0    0                    BaO         25   23   23   23   23   27   27   27   27   27                   PbO         20   0    0    0    0    0    0    0    0    0                    ZnO         10   17   17   17   17   17   12   19   13   10                   B.sub.2 O.sub.3                                                                           5    4    4    4    4    2    6    0    0    5                    TiO.sub.2 + ZrO.sub.2                                                                     5    0    0    0    0    0    0    0    0    0                    Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                       0    0    0    0    0    0    0    0    0    0                    Oxidizing                                                                     agent (%)                                                                     CeO.sub.2   2    0    2    2    2    2    2    2    2    2                    TiO.sub.2   0    0.5  0    2    0    0    0    0    0    0                    BaO.sub.2   0    0    0    0    0    0    0    0    0    0                    SnO.sub.2   0    0    0    0    0    0    0    0    0    0                    CaO.sub.2   0    0    0    0    0    0    0    0    0    0                    V.sub.2 O.sub.5                                                                           0    1.5  0    0    0    0    0    0    0    0                    Properties                                                                    WV*.sup.1 (KV/mm)                                                                         15   17   18   20   24   21   25   25   25   27                   IR*.sup.2 (Ω)                                                                       1 × 10.sup.12                                                                1 × 10.sup.12                                                                5 × 10.sup.12                                                                1 × 10.sup.13                                                                4 × 10.sup.13                                                                3 × 10.sup.13                                                                5 × 10.sup.13                                                                6 × 10.sup.13                                                                2 × 10.sup.13                                                                1 ×                                                                     10.sup.13            DC*.sup.3   8.2  7.2  7.4  7.5  7.8  7.2  7.4  7.9  7.5  7.5                  DCT*.sup.4 (%)                                                                            0.2  0.3  0.1  0.1  0.2  0.3  0.2  0.1  0.1  0.1                  Adh*.sup.5  Good Good Good Good Good Good Good Good Good Good                 Wet*.sup.6 (%)                                                                            100  100  100  100  100  100  100  100  100  100                  Black*.sup.7                                                                              Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil  Nil                  RC*.sup.8 (ppm)                                                                           400  350  270  210  160  150  170  100  180  190                  __________________________________________________________________________    Sample No.   51   52    53   54    55   56    57   58    59                   __________________________________________________________________________    Composition (%)                                                               Alumina      49   49    49   49    49   49    49   49    49                   Zircon       0    0     0    0     0    0     0    0     0                    Forsterite   0    0     0    0     0    0     0    0     0                    Cordierite   0    0     0    0     0    0     0    0     0                    Silica       0    0     0    0     0    0     0    0     0                    Glass        49   49    49   49    49   49    49   49    49                   Glass                                                                         composition (%)                                                               SiO.sub.2    40   38    43   43    40   37    45   43    38                   Al.sub.2 O.sub.3                                                                           5    5     5    5     5    5     8    8     8                    MgO          8    10    0    0     0    0     0    0     0                    CaO          5    5     5    5     5    5     5    5     8                    SrO          0    0     2    5     8    12    0    0     0                    BaO          27   27    27   27    27   27    20   15    18                   PbO          0    0     0    0     0    0     0    0     0                    ZnO          10   10    13   10    10   10    17   20    20                   B.sub.2 O.sub.3                                                                            5    5     5    5     5    5     5    5     5                    TiO.sub.2 + ZrO.sub.2                                                                      0    0     0    2     3    5     0    0     0                    Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                        0    0     0    0     0    0     0    0     0                    Oxidizing                                                                     Agent (%)                                                                     CeO.sub.2    2    2     2    2     2    2     2    2     2                    Properties                                                                    WV*.sup.1 (KV/mm)                                                                          28   29    25   25    26   22    31   30    25                   IR*.sup.2 (Ω)                                                                        2 × 10.sup.13                                                                2 × 10.sup.13                                                                 2 × 10.sup.13                                                                1 × 10.sup.13                                                                 2 ×  10.sup.13                                                               1 × 10.sup.13                                                                 5 × 10.sup.13                                                                5 × 10.sup.13                                                                 1 ×                                                                     10.sup.13            DC*.sup.3    7.7  7.7   7.6  7.7   7.8  7.8   7.4  7.2   7.6                  DCT*.sup.4 (%)                                                                             0.2  0.2   0.1  0.1   0.1  0.2   0.1  0.1   0.1                  Adh*.sup.5   Good Good  Good Good  Good z     Good Good  Good                 Wet*.sup.6 (%)                                                                             100  100   100  100   100  100   100  100   100                  Black*.sup.7 Nil  Nil   Nil  Nil   Nil  Nil   Nil  Nil   Nil                  RC*.sup.8 (ppm)                                                                            200  220   170  160   170  200   150  150   180                  __________________________________________________________________________     *.sup.1 WV: Breakdown voltage                                                 *.sup.2 IR: Insulating resistance                                             *.sup.3 DC: Dielectric constant                                               *.sup.4 DLT: Dielectric loss tangent                                          *.sup.5 Adh: Adhesion to the substrate                                        *.sup.6 Wet: Wettability of the conductor to solder                           *.sup.7 Black: Balckening                                                     *.sup.8 RC: Residual carbon                                              

                  TABLE 2                                                         ______________________________________                                        (Comparative Examples)                                                        Sample No.  1        2        3      4                                        ______________________________________                                        Composition (%)                                                               Alumina     3        68       50     49                                       Zircon      0        0        0      0                                        Forsterite  0        0        0      0                                        Cordierite  0        0        0      0                                        Silica      0        0        0      0                                        Glass        95**     30**    50     49                                       Glass                                                                         composition (%)                                                               SiO.sub.2   43       43       43     35                                       Al.sub.2 O.sub.3                                                                          5        5        5      15                                       MgO         0        0        0      0                                        CaO         5        5        5      5                                        SrO         0        0        0      0                                        BaO         27       27       27     20                                       PbO         0        0        0      10                                       ZnO         15       15       15     0                                        B.sub.2 O.sub.3                                                                           5        5        5      15                                       TiO.sub.2 + ZrO.sub.2                                                                     0        0        0      0                                        Li.sub.2 O + Na.sub.2 O +                                                                 0        0        0      0                                        K.sub.2 O                                                                     Oxidizing                                                                     agent (%)                                                                     CeO.sub.2   2        2         0**   2                                        BaO.sub.2   0        0        0      0                                        CaO.sub.2   0        0        0      0                                        Properties                                                                    WV*.sup.1 (KV/mm)                                                                         15       24       1.1    10                                       IR.sup.*.sup.2 (Ω)                                                                  1 × 10.sup.12                                                                    1 × 10.sup.14                                                                    2 × 10.sup.11                                                                  2 × 10.sup.12                      DC*.sup.3   6.5      7.7      7.3    7.0                                      DCT*.sup.4 (%)                                                                            0.7      0.5      0.7    0.8                                      Adh*.sup.5  Good     Poor**   Good   Good                                     Wet*.sup.6 (%)                                                                            85       100      90     100                                      Black*.sup.7                                                                              Nil      Nil      Yes**  Nil                                      RC*.sup.8 (ppm)                                                                           390      110      2300   230                                      ______________________________________                                         **: Symbol ** indicates a "defect".                                           *.sup.1 WV: Breakdown voltage                                                 *.sup.2 IR: Insulating resistance                                             *.sup.3 DC: Dielectric constant                                               *.sup.4 DLT: Dielectric loss tangent                                          *.sup.5 Adh: Adhesion to the substrate                                        *.sup.6 Wet: Wettability of the conductor to solder                           *.sup.7 Black: Balckening                                                     *.sup.8 RC: Residual carbon                                              

According to the present invention, a green tape having a conductivepaste formed thereon is laminated on a sintered ceramic substrate,whereby the following effects and advantages can be obtained.

1) A conductive paste is applied to a green tape or to an insulatingpaste layer. This is advantageous over a case where the conductive pasteis applied to the sintered ceramic substrate in that the solventcontained in the conductive paste does not diffuse on the surface of thegreen tape or the insulating paste layer, and accordingly a fine patterncan readily be formed. Further, when a resistant paste or the like is tobe formed on the green tape or the insulating paste layer, failure canbe reduced.

2) When a conductive paste is applied to green tapes, the green tapeshaving the conductive paste applied thereon can be inspected beforelamination, and only green tapes found acceptable by the inspection willbe used, whereby the yield can be improved, and the productivity will beimproved accordingly.

3) When an insulating paste layer is formed on the ceramic substrate,the adhesion of the green tape to the ceramic substrate can thereby beimproved.

Further, the green tape or the insulating paste used in the presentinvention not only exhibits excellent electrical properties but alsoexhibits excellent adhesive strength to the ceramic substrate by virtueof the glass component contained in the green tape or in the insulatingpaste, whereby it is possible to provide an excellent multilayer ceramiccircuit board free from interlaminar peeling.

What is claimed is:
 1. A process for fabricating a multilayer ceramiccircuit board which comprises the steps of:(i) providing a sinteredceramic substrate having optionally formed via holes already filled witha conductive paste or a conductive substance; (ii) printing aninsulating paste layer in a prescribed pattern on one side or each sideof the ceramic substrate; (iii) applying a conductive paste in aprescribed pattern on the insulating paste layer on the ceramicsubstrate; (iv) then, providing a ceramic green tape and forming viaholes in the green tape; (v) laminating the green tape to the insulatingpaste layer having a conductive paste in a prescribed pattern thereon;(vi) optionally pressing and heating the laminate from step (v); (vii)applying a conductive paste to the green tape to form a conductivepattern and filling the via holes of the green tape with a conductivepaste, before or after step (v); (viii) laminating to the green tape onthe ceramic substrate a ceramic green tape having via holes formed, andoptionally pressing and heating the laminate; (ix) applying a conductivepaste to the green tape laminated in sep (viii) to form a conductivepattern and to fill the via holes of the same green tape; (x) optionallyrepeating steps (viii) and (ix) until the desired number of circuitlayers has been obtained; and (xi) finally pressing and sintering theassembly of the ceramic substrate and the green tapes.
 2. The processaccording to claim 1, wherein the ceramic substrate is made of alumina,beryllia, MgO or AlN.
 3. The process according to claim 1, wherein theceramic substrate is an alumina substrate containing from 88 to 99.5 wt% of alumina.
 4. The process according to claim 1, wherein the greentape contains glass.
 5. The process according to claim 1, wherein thegreen tape comprises an inorganic component comprising glass, refractoryfiller and an oxidizing agent, and sintering is conducted in asubstantially non-oxidizing atmosphere.
 6. The process according toclaim 1, wherein the green tape comprises an inorganic componentcomprising from 40 to 90 wt % of glass powder, from 9.99 to 60 wt % ofrefractory filler and from 0.01 to 20 wt % of an oxidizing agent,wherein the glass powder consists essentially of from 38 to 48 wt % ofSiO₂, from 1 to 8 wt % of Al₂ O₃, from 0 to 10 wt % of MgO, from 1 to 8wt % of CaO, from 0 to 15 wt % of SrO, from 18 to 28 wt % of BaO, from 0to 20 wt % of PbO, from 10 to 20 wt % of ZnO, from 0.5 to 15 wt % of B₂O₃, from 0 to 7 wt % of TiO₂ +ZrO₂ and from 0 to 5 wt % of Li₂ O+Na₂o+K₂ O, and sintering is conducted in a substantially non-oxidizingatmosphere.
 7. A process for fabricating a multilayer ceramic circuitboard which comprises the steps of:(i) providing a sintered ceramicsubstrate having optionally formed via holes already filled with aconductive paste or a conductive substance; (ii) printing an insulatingpaste layer in a prescribed pattern on one side or each side of theceramic substrate; (iii) applying a conductive paste in a prescribedpattern on any insulating paste layer printed in step (ii) on theceramic substrate; (iv) then, providing a ceramic green tape and formingvia holes in the green tape; (v) laminating the green tape to theinsulating paste layer on one side or each side of the ceramicsubstrate; (vi) pressing and heating the laminate from step (v); (vii)applying a conductive paste to form a conductive pattern on the greentape and filling the via holes of the green tape with a conductivepaste, before or after step (v); and (viii) finally sintering thelaminate.
 8. The process according to claim 7, wherein the ceramicsubstrate is made of alumina, beryllia, MgO or AlN.
 9. The processaccording to claim 7, wherein an insulating paste layer is printed in aprescribed pattern on one side or each side of the ceramic substrate.